Reflector Geometries & Beam Patterns — How UV Reaches the Target

Quick Answer

A reflector typically redirects about 65 % of a lamp's emitted UV back toward the target — without one, the lamp radiates evenly in every direction and only the fraction of the hemisphere that happens to point the right way is used. The geometry you choose decides between a concentrated hot spot (elliptical, good for localised curing) and an even sheet of light (parabolic, good for conveyor applications). The reflector material and its reflectance at the working wavelength matter just as much as the shape.

Why Use a Reflector at All?

A UV lamp is an isotropic emitter — photons leave it evenly in all directions. Without a reflector, roughly half of the output goes into the rear hemisphere, away from the target, and a further share is lost to absorption and scatter on enclosure walls before any of it does useful work.

A well-designed reflector recaptures much of that rear-hemisphere output and redirects it onto the target, substantially increasing the share of "productive" UV photons. This is why most high-power industrial UV systems wrap a reflector around the lamp — it is a direct multiplier on effective installed power.

Exceptions are omnidirectional applications such as in-reactor water disinfection (UV reactors with a quartz sleeve), where the UV is meant to radiate into the medium in all directions anyway.

The Two Main Geometries

Parabolic — Collimated Beam

    Lamp (focal point)
       *
    \  |  /       parallel light flux
     \ | /        downward
   ---+---        (parabola)

The lamp sits at the focal point of the parabola. Every ray that strikes the parabolic wall is reflected parallel downward. The result is an even flux across the full width of the reflector opening.

Use cases:

Conveyor curing (coatings/adhesives): the web passes through and is irradiated evenly across its width
Large-area disinfection of stationary surfaces
Phototherapy

Trade-off: no peak intensity — the energy is spread over the area. If you need peak power (for example, curing a thick adhesive layer in under a second), a parabolic reflector is too weak.

Practical tip: parabolic reflector systems work best at a manufacturer-specified working distance from the target — too close and the beam is not yet collimated, too far and it diverges again.

Elliptical — Focused Line / Spot

    Focus 1
       *         (lamp)
    /     \
   |       |
    \     /
       *         (substrate / target)
    Focus 2

An ellipse has two foci. The lamp sits at focus 1 and the target at focus 2. Geometrically, every ray is concentrated from the first focus onto the second — producing extreme peak intensity along a narrow strip.

Use cases:

High-power curing of thick coatings (medium-pressure mercury lamp, ~200 W/cm)
Spot curing / spot welding in electronics manufacturing
UV crosslinking of adhesives directly at the bond point

Trade-off: a very narrow active strip. For wide webs you must arrange several elliptical reflectors in parallel, or narrow the web.

Maintenance pitfall: accurate positioning of the target at focus 2 is critical — even a few millimetres of offset sharply reduces the peak intensity, because the converging rays no longer meet where the substrate sits.

Hybrid Forms

Off-axis parabolic: a side section of a parabola. Allows non-rotationally-symmetric geometries (for example, UV applied from the side onto a vertical web).
Spherical: simply a half-circle shell. Poor for efficiency (no defined focal point), but inexpensive and used only as an anti-scatter reflector in low-end applications.
Compound (CPC): a compound parabolic geometry — collects rays from a wide acceptance angle onto a narrow exit. Common in solar concentrators, rarer in UV applications.

Material & Reflectance Spectrum

Reflectance at 254 nm (germicidal) is not the same as at visible wavelengths. Material choice is UV-specific:

Material	Reflectance at 254 nm	Notes
PTFE (Spectralon)	~95 %	Lambertian (diffuse), no beam pattern — good for integrating spheres, not for focused geometry
MgF₂-coated aluminium	high	Premium coating, oxidation-resistant, maintains specular UV reflectance; relatively expensive
Enhanced / specular anodised aluminium	high	Industry standard, good price-to-performance balance
Standard anodised aluminium	moderate	Inexpensive; reflectance drops sharply below ~250 nm
Polished bare aluminium (e.g. 6061)	good	Corrosion-prone — the growing oxide layer degrades reflectance over time
Polished stainless steel	low	Generally poor UV reflectance — useful only as an anti-scatter surface, not for performance

Rule of thumb: ordinary anodised aluminium loses a large share of its reflectance in the vacuum-UV range (below ~200 nm). If the application uses 185 nm photons (ozone generation, mineralisation), the reflector material choice is decisive. For a pure 254 nm application the reflector can be cheaper.

Visualising the Beam Pattern

When designing a UV system it helps to actually picture the beam geometry:

Parabolic — even:

Vertical UV intensity across a wide reflector opening looks like a plateau: roughly peak in the centre, still high toward the edges, then a sharp drop-off. A wide active zone with a low peak height.

Elliptical — peaked:

The same lamp power produces a narrow, tall maximum at the second focus: a steep central peak that falls off rapidly to either side. A narrow active zone with a very high peak height.

Without a reflector:

Only the direct lamp output reaches the target — a broad, low 1/r² distribution. As a rule of thumb, whatever you do not recover with a reflector ends up as heat and stray radiation inside the system and must be removed some other way (cooling, light traps).

Practice: Reflectors Age — and Nobody Notices

Reflectors are often forgotten in maintenance documentation. But they degrade in measurable ways:

Oxide layer growth on aluminium reflectors — micro-corrosion produces a dull, greyish film
Dust and aerosol deposits in air applications — these absorb UV and form local hot spots
Water spray and minerals in water applications — scaling
Mechanical deformation from thermal cycling — focal-point drift in elliptical reflectors

Reflector reflectance falls measurably over time, and in many setups it degrades faster than the lamp's own output decay — meaning the reflector can contribute as much to a system's declining UV performance as the lamp itself.

Maintenance best practice:

Re-measure reflector reflectance periodically with a 254 nm radiometer and a test lamp
Replace visibly discoloured reflectors (do not polish them — the surface coating is thin)
Regularly clean the quartz sleeve between lamp and reflector (a fouled sleeve behaves like reduced reflectance)

Without a Reflector — When Does That Make Sense?

Some applications deliberately omit reflectors:

In-reactor water UV systems: the lamp sits in a central quartz sleeve with water flowing around it — the UV is meant to enter the water omnidirectionally, not strike a wall. Here the reactor walls are intentionally UV-absorbing (polished stainless steel's poor UV reflectance is wanted — it stops UV escaping through the wall).
Open-air Far-UVC (222 nm) for room disinfection: the UV is meant to fill the whole room, so directional optics would be counterproductive.
Inline fluid cells for pharmaceutical research: the path is known and the lamp is close to the target, so a reflector would add only marginal performance for a large maintenance burden.

In the large majority of industrial UV applications, however, a reflector is standard and performance-critical.

Cross-References

UV Lamp Anatomy — lamp anatomy as a prerequisite
Ballasts & Drivers — powering the lamp
Excimer Lamps Deep-Dive (coming — 222 nm has its own reflector requirements)
LED vs Mercury — UV LEDs often use integrated lenses instead of classic reflectors
LED Area Emitters — LED area emitters use per-chip reflector cups plus homogeniser lens arrays instead of macro reflectors: a conceptually related but technically distinct secondary-optics architecture

Sources

Last updated: May 2026.